1. Field of the Invention
This invention relates to improvements in a lens meter for measuring two-dimensionally the optical characteristics of a lens to be inspected which is held on a lens receiving plate.
2. Description of the Related Art
Conventionally, there has been known a lens meter in which a lens to be inspected is laid on a lens platform by an inspector, a beam emitted from a light source is then made incident upon the lens, an amount in travel of the beam which has passed through the lens is then detected by a photosensor, and the power at a position on a surface of the lens is measured from the detection result.
In recent years, a progressive multifocal lens and a distance aspheric lens have spread widely as a spectacle lens. With the spread, it has been required to measure the variation of the power at each position on the surface of the lens, that is, to measure the power distribution of the lens. In the conventional lens meter, the lens is successively moved within a plane perpendicular to an optical axis manually by an inspector, and thus the power at the respective positions is read.
Some lens meters of this type have a mechanism for driving the lens with respect to the lens platform. In addition, a lens meter is known in which the two-dimensional power distribution of the lens is measured such that parallel rays of light used as measurement light are projected onto the lens, and moire fringes caused by the travel of the light which has passed through the lens are observed.
However, in the conventional lens meter in which the lens is moved manually by an inspector and then the power at each position of the surface of the lens is measured, there is a problem in that a measuring operation requires much labor and time although the cost of production of the lens is low. In addition, in a lens meter having a driving mechanism for moving the lens automatically within a plane perpendicular to an optical axis, there is a problem in that a mechanical construction becomes complicated and the production costs are raised. In addition, in a lens meter for measuring the power distribution of a lens from the observation of moire fringes, there is a problem in that a lens having a large diameter is required and, worse still, much time is taken to analyze the moire fringes and, as a result, the power distribution cannot be measured promptly.
In order to solve the problems, prior to the present application, the assignee of this invention filed an application for a lens meter, in which a two-dimensionally arranged microlens array used as an optical member for forming many measurement beams of light is disposed between a light source and a lens to be inspected, which is capable of measuring the power distribution at each position of the surface of the lens in a short time and is capable of being constructed at a less cost because the number of light sources to be required is one and a mechanical driving portion is not required (see Japanese Patent Application No. Hei 7-189289, Laid-Open Publication No. Hei 9-33396).
In this lens meter, the lens is mounted on a lens receiving table which is disposed in an optical path between an illumination optical system and a light receiving optical system of a measuring optical system. A beam from the illumination optical system is made incident upon the lens, the beam which has passed through the lens is then received by a photosensor through a light guiding hole formed in the lens receiving table and the light receiving optical system, and the two-dimensional optical characteristics of the lens are measured from a detection result of the photosensor. The axis of the light guiding hole corresponds to the measuring optical axis (the measuring optical center) of the measuring optical system.
In this type of lens meter, a light reception distance on the measuring optical axis from the lens to an optical member of the light receiving optical system or to the photosensor is an essential factor for measuring accurately the optical characteristics of the lens. As the lens to be inspected, use is made of a lens to be machined or processed having a circular shape (i.e., a lens blank), a spectacle lens fitted in a lens frame of spectacles, a spectacle lens of rimless spectacles, or the like.
However, the radius of curvature of a refractive surface of each of these lenses to be machined or processed ranges from a small radius of curvature to a large radius of curvature, depending on the power of the lens. Therefore, when the lens is mounted on the lens receiving table, the light reception distance is varied. This makes it difficult to take an accurate measurement. Further, right and left temples for holding spectacles on the ears make it difficult to mount the spectacles on the lens receiving table in a measurable and stable state.
It might be a possible solution to the problems to dispose the lens platform used in the conventional lens meter on the lens receiving table. Generally, the lens platform is formed in the shape of a cylinder and a frustum, and the diameter at the upper end of the light guiding hole formed inside of the lens platform is made approximately 8.phi. (8 mm). Besides, a beam of light emitted from the light source has the diameter of approximately 6 to 7 mm and is transmitted by the light guiding hole. As a consequence, in the use of the conventional lens platform whose diameter at its upper end is small, the position of the lower surface of the lens to be machined or processed (the circular lens blank) on the measuring optical axis becomes substantially the same as the position of the end of the lens platform even though the radius of curvature of the lower surface of the lens which faces the lens platform is varied. Accordingly, the optical characteristics of the lens can be measured without much difficulty.
However, the lens platform is made from opaque material, and additionally, in the aforementioned lens meter including the microlens array, the diameter of a beam of measurement light is required to be made several times larger than that of the lens platform. Therefore, the conventional lens platform cannot be applied to the lens meter including the microlens array without any improvement. The same can be said of a lens meter for measuring the wide-ranging two-dimensional optical characteristics of a lens to be inspected without any microlens array.